1) Field of the Invention
The present invention relates to a data exchange, a data terminal accommodated in the data exchange, data communication system, and a data communicating method, each suitable for communication systems which tend to be involved in congestion in the high-speed communication such as frame relay networks.
2) Description of the Related Art
Recently, with the introduction of ISDN (Integrated Services Digital Network) or the ITU-T Recommendation, the technology using as a data communication system the frame relay communication system or the packet exchange system has been developed vigorously for variable-length data communications.
In the general packet switching system, the terminal executes a sequential correction or retransmission control with X.25 protocol at an error detecting time while the transit trunk executes a sequential correction or retransmission control with X.75 protocol or datagram control at an error detecting time. Because the packet switching system is utilized at a relatively low communication rate, distributing network resources to each terminal trunk has been inflexibly designed in consideration of only a network configuration time.
The network (frame relay network) adopting the frame relay communication method differs from the X.25/X.75 protocol in the packet network in that the frame discard is permitted for expectation of retransmission from a terminal and the frame discard is performed at the reverse time without performing the sequential correction.
For that reason, the frame relay network enables high-speed communications due to high-speed throughput but tends to be congested. As a result, it is needed to recognize always the status of the load to the communication network and to ensure automatically the most suitable alternate route at a load distribution or congestion occurrence time.
For example, FIG. 41 is a diagram showing a general frame relay network with an alternate circuit. Referring to FIG. 41, numerals 101 and 102 represent frame relay terminals, and 100 represents a frame relay network formed of nodes 103A to 103C. The frame transfer is performed between the frame relay terminals 101 and 102 via the frame relay network 100.
In the frame relay network 100, the nose 103A is connected to the node 103B via the route 104a and to the node 103C via the route 104b. The node 103B is connected to the node 103C via the route 104c.
In order to transmit a frame from the frame relay terminal 101 to the frame relay terminal 102, the node 103A included in the frame relay terminal 101 includes a routing table 108 in which two kinds of routes are decided according to the destination node, as shown in FIG. 42.
When the frame relay terminal 101 transmits a frame to the frame relay terminal 102, the node 103 A first selects the first route 104b to the node 103C acting as a destination node, according to the routing table 108.
If congestion occurs in the first route 104b, the second route 104a is selected. When the congestion is removed in the first route 104b, the route is switched back from the second route 104a to the original first route 104b to perform the frame transfer.
In the general frame relay communication system shown in FIG. 43, the frame relay exchange 103 on the transmitting side includes a high-speed (.about.1.5 Mbps) frame relay terminal 101 and a low-speed (.about.19.2 Kbps) X.25 terminal 105.
The frame relay communication system shown in FIG. 43 permits the X.25 system communication and brings a packet from the X.25 terminal 105 at a high speed via the frame relay transit trunk 106.
However, in the general frame relay network shown in FIG. 41, when the first route 104b and the second route 104a are alternately selected due to the congestion occurrence/congestion removal, the order of original frames sent from the frame relay terminal 101 may be reversed in the destination node 103C. The reversely ordered frame is discarded in the destination node 103C. Hence there are problems in that the congestion occurrence or the frame discard at a congestion occurrence time cannot be sufficiently suppressed to improve the frame transfer efficiency and transfer rate.
Furthermore, in the frame relay communication system shown in FIG. 43, since the transmission queue 107 of the frame relay transit trunk does not have any distinction between the X.25 system and the frame relay system, the communication load from the frame relay terminal whose a subscriber trunk is high-speed greatly affects the delay in the X.25 (packet) communication by the X.25 terminal 105.